The present invention relates to a device for testing a surface of an object with regard to raised areas of the surface, having a light source for producing a beam of light associated with the surface and having a light sensor for detecting light scattered by a raised area of the surface. The present invention also relates to a method for testing a surface of an object with regard to raised areas of the surface, in which a light beam associated with the surface is produced and light scattered by a raised area of the surface is detected.
It is known to test a surface of an object with regard to raised areas of the surface by virtue of the fact that the surface is obliquely illuminated by a light source and observed by an inspector. In a completely general sense, the raised areas of the surface can be impurities which have been deposited on the surface or can be inhomogeneities contained in the surface itself. With appropriate experience, the testing person can recognize, for example, impurities on the order of magnitude of a few micrometers. Clearly, though, this method is only suitable in a limited fashion for the testing of the object in a series production.
In a method known from semiconductor technology, a light beam produced by a light source is directed onto the surface of a silicon wafer at a steep angle. If an impurity is present there, for example, then the light beam is scattered. The presence of the impurity can be inferred from the scattered light that is detected. However, this method can only be used with the extremely flat and smooth surfaces of a silicon wafer.
If the surface of the object to be tested, however, has uneven areas or if the surface is in fact arbitrarily structured, then the light beam striking the surface will be reflected by these uneven areas. The reflected light is then detected in the same manner as the light scattered by impurities. With the known method, however, a distinction cannot be drawn between whether the light has been reflected or scattered. Therefore in this instance as well, the presence of an impurity cannot be reliably inferred from the detected light.
The object of the present invention is to provide a device and method of the type mentioned at the beginning with which uneven or arbitrarily structured surfaces of an object can also be tested with regard to impurities.
This object is attained by the present invention with a device of the type mentioned at the beginning, by virtue of the fact that the light beam produced by the light source is aligned as a side light in relation to the surface in such a way that the raised area of the surface causes a scattering of the light beam, whereas otherwise, the light beam undergoes as little change as possible or undergoes only a slight change, and that the light sensor is disposed in such a way that at least a part of the scattered light can be detected. Furthermore, this object is attained by the present invention with a method of the type mentioned beginning by virtue of the fact that the light beam is aligned as a side light in relation to the surface in such a way that the light beam is scattered by a raised area of the surface whereas otherwise, the light beam is changed as little as possible or is changed only slightly, and that at least a part of the scattered light is detected.
The light beam is therefore directed as a side light onto the surface of the object. If an impurity is present there, for example, then the light beam is scattered and at least a part of the scattered light is detected by the light sensor. The scattered light in this connection can also have components that are produced by means of a possibly subsequent reflection and/or refraction and/or diffraction. The fact that the light sensor is receiving light permits the inference to be drawn that there is an impurity on the surface. The same is true when there are a number of impurities. However, if there is no raised area on the surface, then the light beam passes over the surface of the object without being particularly scattered, reflected, or the like. This also results in the fact that the light sensor does not detect any scattered light. The fact that the light sensor is not receiving light permits the inference to be drawn that there is no raised area and in particular, no impurity on the surface.
It is therefore unnecessary for the surface of the object to be extremely smooth or flat. Even if the surface contains uneven areas or an arbitrary structure, this does not lead to a scattering, reflection, or the like of the light beam. Instead of this, the light beam passes over these uneven areas of the structure without being particularly influenced by them.
Consequently, even when there are uneven or structured surfaces, it is possible to reliably detect raised areas and therefore in particular impurities on the surface of the object through the use of a side light. In this connection, the side light is an easy-to-produce light beam which can also be easily used in the series production of the object.
In advantageous embodiments of the present invention, the light beam produced by the light source is aligned approximately parallel to the surface and/or the light beam produced by the light source has an angle of approximately 0xc2x0 to approximately 10xc2x0 in relation to the surface, in particular an angle of approximately 1xc2x0 to approximately 7xc2x0 in relation to the surface. These embodiments produce a side light that is particularly well-suited on the one hand, to scatter when it strikes against an impurity and on the other hand, to essentially experience no changes otherwise. In particular, with side light embodiments of this kind, uneven areas or structures in the surface to be tested have no particular results with regard to an undesirable scattering, reflection, or the like.
In other advantageous embodiments of the present invention, the light source produces a light beam with approximately parallel light and/or the light beam is slightly divergent. These embodiments produce a side light which is suited to undergoing no changes over a greater length in the event that there are no impurities. Also when there are uneven areas or structures in the surface, essentially no scattering or reflection of the light beam is produced due to the parallel or only slightly divergent light beam.
In an advantageous improvement of the present invention, the light sensor is disposed in a plane that extends approximately perpendicular to the surface and approximately parallel to the light beam. It is particularly useful if the light sensor is disposed approximately in the center above a region that is passed over by the light beam. In this manner, a single light sensor can be used to monitor the entire length over which the light beam passes over the surface. In this improvement, one light sensor is therefore sufficient for testing the surface of the object with regard to impurities.
In another advantageous improvement of the present invention, two or more light sensors are provided, which are disposed three-dimensionally in the space around the surface. It is particularly useful if the light sensors are disposed on the circumference of a cylinder around the light beam. In this manner, it is possible to detect impurities on the surface even better and more reliably. At the same time, this improvement permits the entire method for testing to be accelerated due to the plurality of light sensors.
In another advantageous improvement of the present invention, two or more light sources are provided. It is particularly useful if the light beams of the light sources are aligned approximately parallel to one another and/or if the light beams of the light sources are directed in opposition to one another. In this manner, the light beam can be enlarged with regard to the width with which it passes over the surface to be tested. At the same time, this represents an acceleration of the entire testing method. Furthermore, the fact that the light sources act in opposition to one another results in the better and more reliable detection of impurities on the surface.
Other features, possible uses, and advantages of the present invention ensue from the following description of exemplary embodiments of the present invention which are depicted in the drawing. All of the features described or depicted, alone or in arbitrary combinations, constitute the subject of the present invention, independent of their interrelationship in the claims or their back references and independent of their formulation or depiction in the description or in the drawing.